The present invention relates to a hydraulic motor comprising:
a cylinder block which comprises at least a first group of cylinders disposed radially relative to an axis of rotation, each cylinder being connected to a cylinder duct and co-operating with a piston suitable for sliding in said cylinder;
a reaction member mounted to rotate relative to the cylinder block about the axis of rotation;
an internal fluid distributor constrained to rotate with the reaction member about the axis of rotation and provided with distribution ducts suitable for being connected to the cylinder ducts so as to put said ducts in communication with fluid inlet and fluid outlet ducts of the motor; and
selection means suitable, in a first configuration, for making it possible to connect the cylinder ducts of the cylinders of the first group to the distribution ducts and, in a xe2x80x9cpiston-declutchingxe2x80x9d second configuration, for putting said cylinder ducts in communication with a fluid discharge enclosure, the pistons co-operating with the cylinders of the first group being suitable, in said declutching configuration, for being retracted into said cylinders towards the axis of rotation.
A motor of that type is known from Document FR 2 710 111. When the motor operates at maximum cubic capacity, all of the cylinders are fed periodically with fluid under pressure. As is known, it is possible to interrupt feeding all of the cylinders periodically with fluid so that the motor delivers no torque. As is also known, it is possible to interrupt feeding some of the cylinders with fluid, the other cylinders continuing to be fed periodically with fluid, so that the motor operates at partial cubic capacity.
Thus, the invention is applicable both to a motor in which all of the cylinders belong to the first group and are thus suitable for all being made inactive, and also to a motor further comprising a second group of cylinders, which cylinders continue to be active when the cylinders of the first group are made inactive.
As indicated in FR 2 710 111, it is known that the pistons that are mounted to slide in the cylinders of the first group can be xe2x80x9cdeclutchedxe2x80x9d, an operation which consists in causing said pistons to cease to bear against the reaction member or xe2x80x9ccamxe2x80x9d, thereby eliminating friction and premature wear. For that purpose, a xe2x80x9cdeclutchingxe2x80x9d device makes it possible to maintain the pistons in question in the retracted configuration in which they are retracted inside their respective cylinders.
As also indicated in FR 2 710 111, the declutching operation and the xe2x80x9cre-clutchingxe2x80x9d operation, which consists in putting the pistons back into abutment against the cam, are difficult and, unless certain precautions are taken, they can give rise to the pistons striking the cam violently.
To avoid such violent shocks, FR 2 710 111 recommends using selection means which comprise an individual selector for each of the cylinders of the first group, with the individual selector being controlled in such a manner that, to declutch or to clutch the piston slidably mounted in the cylinder with which it is associated, it can be activated during the relative rotation of the cylinder block and of the cam only while the axis of the cylinder is passing through a position in which it is adjacent to the crest of an undulation on the cam.
That system is quite satisfactory, but it is relatively costly because it requires the presence of as many individual selectors as there are cylinders in the first group. It also needs suitable control means for activating each of the individual selectors at precise times.
Document FR 2 677 409 discloses a hydraulic circuit making it possible to declutch the pistons of a hydraulic motor. That circuit includes a shuttle valve interposed on the ducts that connect the orifice of the pump to the inlet and outlet main orifices of the motor. That system is external, and it is mounted on the motor, which increases the overall size of the motor. In addition, the declutching is applied without exception to all of the pistons of the motor. Finally, the problems of shocks on declutching and on re-clutching are not avoided.
An object of the present invention is to provide a system that is simple and disposed inside the motor and that makes it possible to declutch the pistons slidably mounted in the cylinders of the first group, while substantially avoiding the phenomena of shocks between the pistons and the cam during declutching and re-clutching of the pistons in question, even at speeds of rotation that are relatively high, e.g. about 100 revolutions per minute (r.p.m.) and higher.
This object is achieved by the selection means comprising a single declutching selector constrained to rotate with the cylinder block about the axis of rotation and provided with a communication duct for each cylinder of the first group of cylinders, the selector being suitable for taking up a first position, corresponding to said first configuration, in which the communication ducts make it possible to connect the cylinder ducts of the cylinders of the first group to the distribution ducts, a second position corresponding to said declutching configuration, and an intermediate transitional position between said first and second positions, and in which the cylinder ducts of the cylinders of the first group are connected together via an isolated enclosure.
When the selector is in the first position, the cylinder ducts of the cylinders of the first group are normally connected to the distribution ducts and are thus alternately connected to the fluid inlet and to the fluid outlet, so that the pistons slidably mounted in said cylinders are active. When the declutching selector is in the second position, the cylinder ducts of the cylinders of the first group are all connected to the fluid discharge enclosure so that the pistons slidably mounted in said cylinders are inactive and can be retracted into said cylinders, i.e. they are declutched. Preferably, the fluid discharge enclosure is merely a portion of the inside space of the motor, which portion is connected to a leakage return duct.
When the selector is in the transitional position, the cylinder ducts of the cylinders of the first group are all connected together via an isolated enclosure. The term xe2x80x9cisolated enclosurexe2x80x9d is used to designate a portion of the inside space of the casing of the motor, which portion is isolated from the various xe2x80x9cfunctionalxe2x80x9d ducts of the motor, i.e. the ducts that are connected to ducts external to the motor, such as an inlet duct or an outlet duct, or else an auxiliary duct connected to the booster circuit of the motor to perform an auxiliary function of the motor (braking control, etc.). In other words, the isolated enclosure is connected neither to the inlet nor to the outlet, nor to any auxiliary pressure external to the motor.
In this transitional position, the pistons of the cylinders of the first group can be displaced in their respective cylinders as a function of their positions on the cam. In other words, they are not locked in given positions inside their cylinders, but rather they can follow the undulations of the cam, the volumes of fluid displaced by the retraction or xe2x80x9cinwardxe2x80x9d motion of certain pistons compensating for the volumes of fluid displaced by the outward motion of the other pistons, the motor being constant-velocity.
It is only after the selector has gone through its transitional position that the cylinders of the first group are connected to the fluid discharge enclosure. Thus, the invention makes it possible to avoid putting the fluid discharge enclosure in direct contact with the inlet or the outlet of the motor, which would prevent said motor from operating properly. However, during the displacement of the selector, the cylinder ducts of the cylinders of the first group are not isolated from one another, which avoids locking the pistons slidably mounted in the cylinders.
Advantageously, the declutching selector is formed by a slide mounted to slide in a bore in the cylinder block, the communication ducts comprising communication channels provided in the surface of the slide. The cylinder ducts of the cylinders of the first group have orifices that are open in said bore in the cylinder block, each of the orifices being continuously connected to a respective communication channel; and the bore in the cylinder block is provided with an intercommunication groove to which the communication channels are connected when the declutching selector is in the transitional position.
The communication channels are disposed axially or substantially axially at the surface of the slide. They are thus extremely simple to machine. When the selector is in the first position, the communication channels make it possible to connect the cylinder ducts of the cylinders of the first group with the distribution ducts, so that the pistons of the cylinders of the first group are active. By axially displacing the selector towards its second position, the connection between the distribution ducts and the cylinder ducts of the cylinders of the first group is caused to cease, and the communication channels are all connected to the intercommunication groove in the cylinder block.
The set comprising the cylinder ducts of the cylinders of the first group, the communication channels, and the intercommunication groove constitutes the isolated enclosure via which the cylinder ducts of the cylinders of the first group communicate with one another when the selector is in the transitional position.
In which case, advantageously, for each cylinder duct of the cylinders of the first group, the motor has a distribution passageway formed by a duct that has a first orifice open in the bore of the cylinder block and a second orifice open in a communication face of the cylinder block so as to be connected to the distribution ducts during the relative rotation of the cylinder block and of the distributor, and, for each cylinder duct of the cylinders of the first group, the first orifice of the duct forming the distribution passageway, the orifice of the cylinder duct and the intercommunication groove are successively disposed in the bore of the cylinder block in the direction in which the slide of the selector is displaced from its first position to its second position.
For each cylinder of the first group, the communication passageway is connected continuously to the communication face of the cylinder block and it can thus be connected alternately to each of the distribution ducts. For each cylinder duct of the cylinders of the first group, the communication channel establishes the communication between the cylinder duct and the distribution passageway, when the selector is in the first position. When the selector is displaced into its second position, the communication between the distribution passageway and the cylinder duct ceases, while said cylinder duct is connected to the intercommunication groove, also via the communication channel.
Advantageously, for causing the declutching selector to be displaced between its first and second positions, the motor further comprises control means which comprise resilient return means suitable for continuously urging said selector towards one of its first and second positions, and counter-force means suitable for being controlled so as urge the selector towards the other of its first and second positions.
In a variant, the resilient return means urge the selector towards its first position, and the counterforce means comprise a control piston co-operating with a control cylinder secured to a stator portion of the motor, said control piston being suitable for being displaced against the return force of said return means so as to urge the selector towards its second position.
The declutching of the pistons of the cylinders of the first group is then obtained by a positive command of the control piston, against the resilient return means, this command being, in particular, hydraulic.
It is possible to make provision for the control piston to co-operate with the selector via spherical abutment means, or via ball or roller bearing means, or via hydrostatic abutment means using a confined fluid.
The control cylinder is secured to a stator portion of the motor, while the selector is constrained to rotate with the cylinder block. Therefore, that portion of the selector with which the control piston co-operates rotates relative to the control cylinder. The choice of spherical abutment means, of ball or roller means, or of hydrostatic abutment means makes it possible to avoid premature wear of the parts that rotate relative to one another. In particular, choosing spherical abutment means makes it possible to limit the friction between the control piston and the selector to a zone of small area. This zone is preferably aligned with the axis of rotation of the motor, both the cylinder block bore in which the declutching selector is disposed, and also the control cylinder preferably being centered on the axis of the motor. Thus, the relative speed between the stationary part and the rotary part is low, so that, at the most, very low torque is generated in this zone. As a result, the friction, and thus the heating of the parts in contact are limited.
Choosing the ball abutment means makes it possible substantially to eliminate the friction by replacing it with a rolling contact, thereby also avoiding risks of friction and thus of premature wear.
When hydrostatic abutment means are chosen, the contact friction is also eliminated and is replaced by fluid friction, which offers substantially the same advantages.
In another variant, the counter-force means comprise a control chamber provided between the selector and a reference part fixed relative to the cylinder block, and a control duct suitable for connecting said control chamber to a fluid source.
It is possible to make provision for the resilient return means to urge the selector towards its second position, and for the counter-force means to be suitable for being controlled to urge the selector towards its first position.
In which case, the declutching configuration is obtained at rest, by the resilient return means. This is advantageous, for example, when the motor serves to drive a vehicle. If a fault in controlling the counter-force means occurs while the vehicle is being driven with the pistons in the clutched position (thus at low speed and at high motor torque), this fault causes the selector to go into the declutched configuration, thereby reducing the cubic capacity and thus the torque of the motor. The driver then has no difficulty in controlling the speed by controlling the flow rate of the pump.
This variant is also advantageous in a hydraulic motor equipped with a parking and safety brake. In which case, if a user tries to start the hydraulic motor without having released the braking, the braking torque necessary to prevent untimely driving of the motor and of the vehicle equipped with the motor must merely be capable of overcoming the torque developed by the motor in its declutched configuration (i.e. at low cubic capacity) which is the rest configuration. This braking torque is less than the torque that would be necessary if the rest position of the selector were its first position (in which the pistons are clutched), which makes it possible to choose a brake that is more compact and less costly.
In an advantageous layout, a decompression chamber is provided at that end of the declutching selector which is situated on the downstream side thereof in the direction in which it is displaced from its first position to its second position, and, when said selector is in its second position, the cylinder ducts are connected to said chamber.
When the selector goes from its first position to its second position, the decompression chamber is fed with the fluid that is situated in the cylinder ducts of the cylinders of the first group. The decompression chamber is thus xe2x80x9cput under pressurexe2x80x9d, thereby applying a force opposing the continued displacement of the selector towards its second position. In other words, the displacement of the selector between its transitional position and its second position is slowed down, so that the declutching takes place smoothly.
This decompression chamber is advantageously in communication with a leakage return duct connected to a pressure-free tank. This communication advantageously takes place via at least one passageway of small cross-section, thereby causing a head loss between the decompression chamber and the leakage return duct, which head loss makes it possible to prevent the selector from being displaced too quickly from its first position to its second position.
The size of the through cross-section of the passageway(s) of the small cross-section determines the head loss between the decompression chamber and the leakage return duct, thereby conditioning the speed of displacement of the selector between its transitional position and its second position.
The invention is applicable to motors in which all of the pistons are to be declutchable. It is also applicable to motors that each have at least two distinct operating cubic capacities and in which only the pistons of those cylinders which are inactive at low cubic capacity(ies) are to be declutched.
In which case, advantageously, the cylinder block has a second group of cylinders disposed radially relative to the axis of rotation, each cylinder of the second group being connected to a respective cylinder duct which is connected directly to a communication face of the cylinder block, which face co-operates with a distribution face of the distributor, so as to be connected to the distribution ducts during the relative rotation of the cylinder block and of the distributor, independently of the position of the declutching selector.
Thus, when the pistons slidably mounted in the cylinders of the first group are declutched, those which are slidably mounted in the cylinders of the second group can, insofar as said cylinders continue to be connected to the distribution ducts, continue to be active to deliver a low motor torque. In which case, the motor operates at a low cubic capacity, whereas, when the selector is in its first position, all of the cylinder ducts are connected to the distribution ducts, and it operates at a high cubic capacity.
It should be noted that, in addition to the selector for declutching the pistons, it is possible to provide an additional cubic capacity selector that acts on the connection between the cylinder ducts and the distribution ducts. This cubic capacity selector may be controlled independently of the declutching selector so as to be caused to go between two positions corresponding respectively to a higher cubic capacity and to a lower cubic capacity. It is thus possible to obtain four distinct operating cubic capacities depending on whether the selectors are simultaneously in their first positions or in their second positions, or whether one is in its first position while the other is in its second position.